This invention relates to the field of optical communication. More particularly, this invention relates to the field of optical communication where there is a need to measure wavelength jitter.
In WDM (wavelength division multiplexing) optical communication, multiple wavelengths of light each carry a communication signal. Each of the multiple wavelengths of light forms a channel. In DWDM (dense WDM) optical communication, a subset of the WDM optical communication, the channels are spaced close together. A typical DWDM application operates at a wavelength band about 1,550 nm, has 40 channels, and has spacing of 0.4 nm between adjacent channels.
In the WDM optical communication there is a need to monitor the wavelength of each channel. This is especially required for the DWDM optical communication because of the close spacing between adjacent channels.
A method of monitoring wavelength of each of the channels employs a scanning source, a Michelson interferometer, and a detector. The scanning source individually directs each of the channels sequentially to the Michelson interferometer. The Michelson interferometer sequentially directs interference fringe patterns corresponding to each of the channels to the detector. The detector detects the interference fringe pattern, which are electronically compared to desired interference fringe patterns.
There are a number of problems associated with this method. An instrument employing this method is expensive. Further, because the method employs a scanning source, the method exhibits a slow update rate. Moreover, as the channel spacing has decreased in the DWDM optical communication, a dynamic range provided by the method is proving to be insufficient.
The Michelson interferometer is an amplitude splitting interferometer which provides information about light which enters it. The Michelson interferometer includes a beam splitter, first and second mirrors, and a focusing lens. Light entering the Michelson interferometer is split by the beam splitter into a transmitted light and a reflected light. The reflected light then reflects from the first mirror and returns to the beam splitter. Meanwhile the transmitted light reflects from the second mirror and returns to the beam splitter. The beam splitter combines the transmitted light and the reflected light into interference light which is focused by the focusing lens into an interference pattern. As wavelength of the light changes the interference pattern changes.
A Mach-Zehnder interferometer is an amplitude splitting interferometer which provides information about an object placed within a first leg of the Mach-Zehnder interferometer rather than information about light entering the Mach-Zehnder interferometer. For example, the Mach-Zehnder interferometer is often used to observe a gas flow pattern caused by density variations in the gas flow pattern. The first leg of the Mach-Zehnder interferometer passes through the gas flow patterns while a second leg of the interferometer does not pass through the gas flow pattern. First light, diffracted by the gas flow pattern within the first leg, interferes with second light which traverses the second leg. A focusing lens produces an image of the gas flow pattern by focusing the first and second light in an image plane.
What is needed is a method of monitoring channel wavelength that is less expensive.
What is needed is a method of monitoring channel wavelength that exhibits a better update rate.
What is needed is a method of monitoring channel wavelength that exhibits a better dynamic range.
What is needed is a Mach-Zehnder interferometer which provides information about light entering the Mach-Zehnder interferometer rather than information about an object within a leg of the Mach-Zehnder interferometer.
An apparatus for detecting wavelength change of a first light signal of the present invention comprises an amplitude splitting interferometer and a detector. The amplitude splitting interferometer comprises first and second optical paths. The first optical path has a first index of refraction that varies with wavelength over a first wavelength band. The second optical path has a second index of refraction that is relatively constant over the first wavelength band. In operation the first light signal enters and exits the amplitude splitting interferometer forming interference light. The interference light couples to the detector which detects the wavelength change of the first light signal from the interference light.
An interferometer of the present invention comprises a first beam splitter, third and fourth optical paths, and a second beam splitter. The third optical path is optically coupled to the first beam splitter and has a third index of refraction that varies with wavelength over a second wavelength band. The fourth optical path is optically coupled to the first beam splitter and has a fourth index of refraction that is relatively constant over the second wavelength band. The second beam splitter is optically coupled to the first and second optical paths such that in operation an incident light enters the first beam splitter and exits the second beam splitter forming an output light and further such that in operation a change in wavelength of the incident light within the wavelength band causes a change in interference of the output light.